Method for low loss control of a capacitive load, in particular of a piezoelectric actuator

ABSTRACT

The invention related to a method for low loss periodic control of a capacitive load (C P ), in particular of a piezoelectric actuator. In this method, before load control begins, a capacitor bank (C 1 ) is charged via a voltage source (U B ) until the oprating voltage of load (C P ) is attained; then the load (C P ) is charged from the capacitor bank (C 1 ) via a series resonant circuit formed by a reactance coil (L 1 ), a freewheeling diode (D E ), and by the load (C P ). In this method, when a load is required, the load (C P ) is linked via discharge switch (S E ) to the reactance soil (L 1 ) which is itself connected via a freewheeling freewheeling diode (D L ) to the capacitor bank (C 1 ) in such a way that the load (C P ) is discharged until a residual voltage (U R ).

BACKGROUND OF THE INVENTION

The invention relates to a method for low loss control of a capacitiveload C_(P), particularly a piezoelectric actuator.

With respect to electrical control, piezoelectric actuators behave inthe same way as capacitors, which are charged either from a currentsource or a voltage source. Piezoelectric actuators are normallyactuated via a so-called RC control. With this method, the piezoelectricactuator is charged from a voltage source, e.g. a large capacitor, byway of a switch and an ohmic resistance. The switch is opened for thedischarging and a second switch is closed, subsequently short-circuitingthe piezoelectric actuator via another ohmic resistance. The chargingcurve and thus also the elongation curve of the piezoelectric actuatorcan be selected freely owing to the choice of resistances. The greatdisadvantage of the RC control is the high power consumption of largeactuators, meaning for a correspondingly high capacitance and highoperating frequencies. In the process, the energy W=CU is converted inthe resistances and the actuator into dissipation heat during eachcharging cycle and discharging cycle.

SUMMARY OF THE INVENTION

It is the object of the invention to develop a method for controllingpiezoelectric actuators, which method for the most part reduces theelectrical losses. That is, it is an object of the invention to developa low-loss, and therefore energy-saving, circuit for activatingpiezoelectric actuators.

This object is solved according to the invention in that at the start ofthe load control, a capacitor bank is charged from a voltage source tothe operating voltage of the capacitive load C_(P), that the load C_(P)is charged from the capacitor bank via a series resonant circuit formedby a reactance coil of a freewheeling diode and the load C_(P). If aload is required in this case, the load C_(P) is connected via adischarge switch to the reactance coil, which is connected via afreewheeling diode to the capacitor bank, so that the load C_(P) can bedischarged, except for a residual voltage. The advantage of this methodis that when switching the load case, the residual load initiallyremains on the capacitive load C_(P) and that subsequently this residualload can be conducted back to the capacitor bank, so that the residualload for the most part can be reclaimed.

According to one embodiment of the method according to the invention, itmakes sense to briefly connect the reactance coil to a ground to reclaimthe residual load from the capacitive load C_(P), so that the residualload can be discharged to the capacitor bank. As a result of connectingthe reactance coil briefly to ground, it is possible to regain thecomplete reactive power that was stored in the load.

Based on another embodiment of the method according to the invention, itis advantageous and particularly useful if the capacitor bank is muchlarger than the capacitive load C_(P). In this way, a maximum voltagetransformation ratio, e.g. with factor 2, can be achieved between thesource voltage and the load voltage. Above all, this is the case if thecapacitor bank capacitance is practically infinite in relation to thecapacitance of load C_(P).

Above all, the method according to the invention has the particularadvantage that the full load operation voltage from the capacitor bankis available after the capacitor bank is charged to the operatingvoltage. Adjusting the voltage at the capacitor bank to a stable initialstate permits a stable and reproducible charging of the capacitive loadC_(P). The voltage at the capacitor bank as well as the voltage at theload C_(P) can be used as input variable for the adjustment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail with the aid of switchingdiagrams and current and voltage diagrams, in which:

FIG. 1 is switching arrangement based on the state of the technology;

FIG. 2 is switching arrangement for implementing the method according tothe invention; and

FIGS. 3a-d illustrate the time curves for control pulses voltage at thecapacitor bank C₁, voltage at the load C_(P), and load current,respectively.

DETAILED DESCRIPTION OF THE INVENTION

The switching arrangement shown in FIG. 1 for implementing the methodaccording to the state of the technology contains a piezoelectricactuator C_(P), which can be charged by way of a capacitor bank C₁. Thecapacitor bank C₁ is connected to a corresponding, equal current supplyU_(B), which serves to recharge the capacitor bank C₁. The actuatorC_(P) is charged by closing a switch S₁ via an ohmic resistance R₁. Theswitch S₁ is opened for the discharging and a second switch S₂ isclosed, which then short circuits the actuator C_(P) via a second ohmicresistance R₂. Owing to the section of the resistances R₁ and R₂, thecharging curve and thus also the elongation curve of the actuator C_(P)can be selected freely. The initial charging of the actuator C_(P) thusoccurs directly from the capacitor bank C₁. If the actuator C_(P) isdischarged, a considerable residual charge remains on the actuatorC_(P), which is destroyed with the aid of a short circuit.

The switching arrangement shown in FIG. 2 for implementing the methodaccording to the invention essentially consists of a same voltage sourceU_(B), which is connected via a diode D_(v) to the capacitor bank C₁with much higher capacitance than the capacitive load of the actuatorC_(P). By closing a load switch S_(L) and an inductance L₁, the actuatorC_(P) can be charged via the diode D_(E).

Following the charging operation, the load switch S_(L) is opened, asshown, and a discharge switch S_(E) is closed, so that the dischargingcan occur via the inductance L₁ and a diode D_(L) that is locked ontothe capacitor bank C₁. As a result, the actuator C_(P) charge for thecapacitor bank C₁ can for the most part be reclaimed.

The residual charge remaining after the discharging operation as aresult of the internal resistance of actuator C_(P) is stored in theinductance L₁ by briefly closing a residual discharge switch S_(R).Following the opening of the residual discharge switch S_(R), theresidual charge is fed back to the capacitor bank C₁ where it is stored.The advantage of this is that the capacitor bank C₁ must be rechargedonly with the low dissipation energies from the voltage source U_(B). Itmakes sense if the direct voltage source U_(B) and the capacitor bank C₁are combined to form a controlled voltage source, wherein this voltagesource is configured such, with respect to the regulation, that therespectively desired voltage can be adjusted to a respectively desirablevalue at the actuator C_(P).

The above described charging and discharging operation is explained infurther detail in the following, relative to dependence in time, andwith the aid of the diagrams shown in FIGS. 3a-d.

In this case, the diagram 3 a) shows the sequence and duration of thecontrol pulses. The diagram 3 b) shows the course over time of thevoltage U_(C) at the capacitor bank C₁, while the diagram 3 c) shows thecurve for the load voltage U_(P). The diagram 3 d) shows the loadcurrent curve.

A charging and discharging operation is explained with the aid of thesediagrams. Prior to the start of the load control, meaning prior to thepoint in time T₀, the capacitor bank C₁ is charged to the voltage U_(B)via the diode D_(V). At point in time T₀, the load interrupter switchS_(L) is closed. The reactance coil L₁ forms a series resonant circuitwith the capacitive load C_(P) via the freewheeling diode D_(E). Thecharge is applied to the capacitive load. The voltage at the capacitorbank C₁ drops and the voltage at the load C_(P) increasescorrespondingly. The current curve has a sine shape. The chargingoperation is completed following the time interval t₁. If the loadcircuit remains closed past this time interval, up to the point in timeT₂, then this does not have an effect on the voltage at the load sincethe diode D_(E) prevents a reversal of the current direction.

The load interrupter switch S_(L) can be opened at any time, followingcompletion of the charging operation. The point in time for opening isnot critical in this case. If the load is to be discharged at point intime T₃, then the discharge switch S_(E) is closed. The charge flows viathe reactance coil L₁ and the freewheeling diode D_(L) back into thecapacitor bank C₁. The current curve in this series resonant circuitagain has a sine shape. The voltage at load C_(P) has dropped to theresidual voltage U_(R) during the time interval t₄. Actuating thedischarge switch S_(E) past this time interval does not result in avoltage reduction at the capacitive load. The voltage increase at thecapacitor bank C₁ reflects the energy feedback.

In addition to the discharge switch S_(E), the residual discharge switchS_(R) is closed at point in time T₅. The reactance coil L₁ then forms anew series resonant circuit together with the capacitive load C_(P). Theresidual energy stored in the capacitive load C_(P) is thus stored inthe reactance coil L₁. The voltage at the load C_(P) has died down to 0at point in time T₆. The discharging operation of load C_(P) is thuscompleted. The current in the reactance coil L₁ is therefore at amaximum. The residual discharge switch S_(R) must be opened at thatmoment. If the residual discharge switch S_(R) remains closed, thefreewheeling diode D_(P) becomes conductive. The load voltage is clampedonto the forward voltage of the freewheeling diode D_(P). This resultsin losses at the path resistance for diode D_(P) and the ohmicresistances of the resonant circuit. The discharge switch S_(E) thusmust be opened, if possible, immediately after discharging thecapacitive load C_(P).

Following the opening of the residual load interrupter switch S_(R) atpoint in time T₆, the current flow is maintained through reactance coilL₁, via the freewheeling diodes D_(L) and D_(P) and the discharge switchS_(E) and onto the capacitor bank C₁. In this way, the residual energypreviously stored in the load C_(P) is released to the capacitor bankC₁. The voltage at capacitor bank C₁ increases again. At point in timeT₇, the reactance coil L₁ is without energy. The feedback operation andthus the complete switching cycle are concluded. The point in time T₈for opening the discharge switch S_(E) is not critical. The capacitorbank C₁ is recharged from the source to the voltage U_(B).

The diodes D_(L) and D_(E) can be omitted if the control for switchesS_(L) and S_(E) are laid out in such a way that these are actuated atthe correct point in time. The herein suggested solution with diodesD_(L) and D_(E) has the great advantage that these “select” practicallyby themselves the “correct” point in time for switching and anexact-time control of the discharge switch S_(E) during the charging andthe load switch S_(L) during the discharging can be omitted.

What is claimed is:
 1. A method for low-loss, periodic control of acapacitive load C_(P) comprising the steps of: prior to the start ofload C_(P) actuation, charging a capacitor bank C₁ to an operatingvoltage from a voltage source U_(B); charging the load C_(P) withvoltage from the capacitor bank C₁ via a resonant circuit formed by areactance coil connected in series with a first freewheeling diode D_(E)and the load C_(P); and when voltage from load C_(P) is required,connecting the load C_(P) via a load switch S_(E) to the reactance coilL₁ and then via a second freewheeling diode D_(L) to the capacitor bankin order to discharge the voltage at load C_(P), said secondfreewheeling diode D_(L) being connected in series with the reactancecoil L₁ and in parallel with the capacitor bank C₁.
 2. The methodaccording to claim 1, wherein the connecting step leaves a residualvoltage U_(R) at load C_(P) and further comprising the step of brieflyconnecting the reactance coil L₁ to ground in order to reclaim theresidual voltage U_(R) from load C_(P) to further discharge the voltageat load C_(P).
 3. The method according to claim 1, wherein thecapacitance of the capacitor bank C₁ is higher than the capacitance ofthe load C_(P).
 4. The method according to claim 1, wherein thecapacitive load C_(P) is a piezoelectric actuator.
 5. A circuit forlow-loss, periodic control of a capacitive load C_(P) comprising: acapacitor bank C₁ connected in parallel with a voltage source U_(B); aresonant circuit connected in parallel with the capacitor bank C₁ andformed by a reactance coil L₁ connected in series with a first diodeD_(E) and the capacitive load C_(P); and a discharge switch S_(E)connected in parallel with said first diode D_(E) and connected in aseries with said reactance coil L₁ and said capacitive load C_(P),wherein the capacitor bank C₁ is charged to an operating voltage fromthe voltage source U_(B), the capacitive load C_(P) is charged from thecapacitor bank C₁ via the series resonant circuit when the dischargeswitch S_(E) is open, and the capacitive load C_(P) is discharged whenthe discharge switch S_(E) is closed.
 6. The circuit according to claim5, where in the capacitive load C_(P) is a piezoelectric actuator. 7.The circuit according to claim 5, further comprising a second diodeD_(L) connected between said capacitor bank C₁ and said reactance coilL₁, and a load switch S_(L) connected in parallel with said second diodeD_(L), wherein when said discharge switch S_(E) is closed and the loadswitch S_(L) is open, the load C_(P) is connected to the capacitor bankC₁ via the reactance coil L₁ and the second diode D_(L) therebydischarging the capacitive load C_(P), except for a residual voltaicU_(R).
 8. The circuit according to claim 7, wherein the first and seconddiodes are free-wheeling diodes.
 9. The circuit according to claim 7,further comprising, a residual switch S_(R) connected in parallel withsaid capacitor bank C₁, wherein the reactance load is briefly connectedto a ground via a residual switch to order to discharge the residualU_(R) from the load C_(P).
 10. The circuit according to claim 5, whereinthe capacitor of capacitor bank C₁, is much higher than the capacitanceof the load C_(P).
 11. The circuit according to claim 9, therein thecapacitance of the capacitor bank C₁ is much higher than the capacitanceof the load C_(P).